ECMAScript 2015 Promise polyfill with Fiber support

The static methods `Promise.async` and `Promise.await` implement the
relaxed `async` and `await` functions proposed in this talk: [Why Fibers
Make Sense For Meteor](http://benjamn.github.io/goto2015-talk).

If you'd like to use this [package](https://atmospherejs.com/meteor/promise)
in a [Meteor](https://www.meteor.com/) project, a much simpler
approach is simply to run `meteor add promise`.
